Ultra-fast science succeeds at SLAC

The electron bunch alters the electro-optic crystal (vertical line) when it passes by the crystal. Laser light (diagonal line) that shines through the altered crystal is also affected, recording the electron bunch length and arrival time.
Image by Adrian Cavalieri (Univ. Michigan)
Click here for a high resolution photograph.

The Sub-Picosecond Pulse Source (SPPS) collaboration has published data from the first experiments ever using a linear accelerator-based femtosecond x-ray source, and has developed an important tool for ultra-fast science. SPPS makes the world’s shortest bunches of electrons in the SLAC linear accelerator and turns them into very bright pulses of x-ray light one thousand times shorter than those made in synchrotron rings like SPEAR3.

Pulses of such short duration—lasting some 80 femtoseconds, or 80 millionths of billionths of a second—shine a lightning-fast strobe light on the swift movements of the microscopic milieu.

“Because of the amazing properties of the x-ray source, we were able to answer a long-standing problem in condensed matter physics, concerning how solids transform into liquids on ultra fast time scales,” said Aaron Lindenberg (ESRD).

Lindenberg was the lead author on the SPPS paper published April 15 in Science. The other paper, headed by Adrian Cavalieri (University of Michigan), appeared in the March 25 Physical Review Letters; the researchers used SPPS to develop and test a new timing technique, which will be essential for many SPPS and Linac Coherent Light Source (LCLS) experiments. Both papers involved collaborators from multiple institutions around the world.

“Since SPPS has so many similarities to future free electron lasers like LCLS, currently being built at SLAC, these experiments lay the groundwork for the next generation of ultra-fast experiments,” Lindenberg said.

The laser light (bottom image) initiates the transition of a material from solid to liquid while the scattered x-rays provide a glimpse of the first transitional step. The upper diffraction pattern (top image) results when no laser light shines on the sample. The lower diffraction pattern shows the sample at various times before and after being struck by laser light.
Image by Aaron Lindenberg (SSRL)
Click here for a high resolution photograph.

The new Ultra-fast Science Center at SLAC, Stanford and SSRL will provide world leadership in ultra-fast research (including experiments at SPPS and LCLS) and the development of experimental techniques.

In the first SPPS experiment, researchers shone laser light to melt a room temperature crystal of semiconductor material, and sent x-ray pulses to probe the material. The scattered x-rays provided a glimpse of the first step in the transition from solid to liquid. In those first few hundred femtoseconds between solid and liquid, the atom positions had on average the crystalline (regular, repeated) structure of a solid, yet the atoms had moved far from their initial positions, with a disordered structure like a liquid.

“It’s the first time we’ve been able to watch the pathways the atoms follow in the first femtoseconds as the material transitions from solid to liquid,” he said.

Initially, atoms randomly move small distances as they vibrate, but are kept in position by chemical bonds to other atoms. The laser light instantaneously broke the bonds, allowing atoms to continue moving in the random direction they were headed just before the bonds broke. This takes place before the atoms heat up because the time scale is faster than the time it takes to transfer energy from the laser to atoms in the crystal. The result is a very unusual, intermediate state of matter.

Researchers learned the transition state is governed by inertial dynamics, simply stated by Newton’s First Law as: an object in motion continues in motion (in the same direction). Understanding the transition steps of ultra-fast melting may have technical applications, for example in micro machining and laser eye surgery.

Clocking Femtosecond X-rays

In ultra-fast experiments, timing is everything. The other SPPS experiment solved a major issue by borrowing ideas from ultra-fast laser technology. Many SPPS and LCLS experiments will require a laser to pump, or start, a process in the system under investigation. To put data in order chronologically—important for seeing chemical or other reactions over time—researchers need to time-stamp the arrival of the laser pulse and the arrival of the x-ray pulse that probes, or observes, the system.

Cavalieri and his collaborators used electro-optic sampling to measure the arrival time of the x-ray pulses in relation to the arrival of the laser pulses. The laser pulses travel a few feet to the experimental sample, while the x-ray pulses originate as electron bunches two miles away at the start of SLAC’s accelerator. And while laser pulses can be put out in steady, reliable intervals, it’s tricky to perfectly time the electron/x-ray beam, so there is an intrinsic time jitter, where each x-ray pulse arrives at a slightly different time relative to the laser pulses.

Just before an electron bunch gets converted into an x-ray pulse, it speeds past an electro-optic crystal placed next to the beam. The strong electric field generated by each electron bunch alters the properties of the crystal, but only at the instant the electrons pass by. Experimenters then use an ultra-fast laser pulse to probe this change. The characteristics of the laser light exiting the crystal reveal the electron bunch length and arrival time, which in turn indicates the arrival time of the corresponding x-ray pulse.

“The angle of the laser pulse sweeping through the electro-optic crystal changes space into time. The geometry fixes the sweep rate and the time window,” Cavalieri said.

To confirm the technique’s reliability, scientists plotted their electro-optic timing data against the timing data from the SPPS melting experiment—a rare case where the signal strength allowed data collection at all time intervals in a single shot—and found good agreement.

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